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Silicon (Si) is a pivotal element in the biogeochemical and ecological functioning of the ocean. The marine Si cycle is thought to be in internal equilibrium, but the recent discovery of Si entries through groundwater and glacial melting have increased the known Si inputs relative to the outputs in the global oceans. Known outputs are due to the burying of diatom skeletons or their conversion into authigenic clay by reverse weathering. Here we show that non-phototrophic organisms, such as sponges and radiolarians, also facilitate significant Si burial through their siliceous skeletons. Microscopic examination and digestion of sediments revealed that most burial occurs through sponge skeletons, which, being unusually resistant to dissolution, had passed unnoticed in the biogeochemical inventories of sediments. The preservation of sponge spicules in sediments was 45.2 ± 27.4%, but only 6.8 ± 10.1% for radiolarian testa and 8% for diatom frustules. Sponges lead to a global burial flux of 1.71 ± 1.61 TmolSi yr−1 and only 0.09 ± 0.05 TmolSi yr−1 occurs through radiolarians. Collectively, these two non-phototrophically produced silicas increase the Si output of the ocean to 12.8 TmolSi yr−1, which accounts for a previously ignored sink that is necessary to adequately assess the global balance of the marine Si cycle.

https://www-iuem.univ-brest.fr/lemar/wp-content/uploads/2019/08/ADM-eponges-3.jpg4231210sherve@univ-brest.frhttps://www-iuem.univ-brest.fr/lemar/wp-content/uploads/2018/10/logo-lemar-big.pngsherve@univ-brest.fr2019-08-30 14:41:272019-09-12 14:27:09Sponge skeletons as an important sink of silicon in the global oceans

It is widely recognized that the emergence and expansion of silica biomineralization in the oceans has affected evolutionary competition for dissolved Si (DSi). This resulted in changes in the global biogeochemical cycles of silica, carbon (C) and other nutrients that regulate ocean productivity and ultimately climate. However, a series of very recent discoveries in geology and biology suggest that the first biological impacts on the global Si cycle were likely by prokaryotes during the Archean with further decreases in oceanic DSi with the evolution of widespread, large-scale skeletal biosilicification significantly earlier than the current paradigm. Our project interweaves geology and biology and will create new knowledge into the interactions between biosilicification in organisms and the environment and how these interactions have evolved through Earth’s history. Together, these geological and biological analyses will provide novel insights into the key events during periods of DSi drawdown, which reorganize the distribution of carbon and nutrients, changing energy flow and productivity in the biological communities of the ancient oceans.

Better understand and quantify the oceanic biogeochemical cycles of major elements and the biological carbon pump.

The study of the trace metals cycle is one of LEMAR’s strong themes. Improving our knowledge of the metal cycle is crucial to better understand and quantify oceanic biogeochemical cycles of major elements (C, Si, N, S) and the biological carbon pump. The analysis of trace metals and their speciation is particularly difficult because their concentrations are extremely low and their cycle is complex. LEMAR is one of the internationally recognized laboratories for the study of the trace metals cycle, notably through the use and development of advanced techniques (SF-ICP-MS in the context of PSO, FIA, voltammetry). Our expertise in both the dissolved and particulate phase will allow us to study the interactions between these two reservoirs, notably at the oceanic interfaces. These interactions are very little studied at present and yet fundamental to better understand the bioavailability of metals. This theme will strengthen our international visibility, particularly in the context of new GEOTRACES oceanographic campaigns.

The coastal zone, at the interface between the earth system and the sea system, concentrates a set of interfaces and natural environmental gradients, generating a very strong heterogeneity at different spatio-temporal scales. Scientific questions are therefore numerous to try to better understand the nature and dynamics of physical, biological and geochemical flows and forcings, and their interactions and feedbacks (prospective SIC-INSU). Extremely dynamic and complex, this coastal zone is also the seat of many facets of global change with climate change of course, but also strong and growing anthropogenic pressures related to urban planning, land use, exploitation mineral and living resources on land and at sea. In this context, our objectives are threefold:

● develop an integrated approach to land-to-sea transfers of dissolved and particulate matter, combining observation, process studies and modeling in estuaries and coastal areas to better understand the coastal ecosystem’s response to physical, biogeochemical forcings and biological, terrestrial and oceanic (Axis 1 of Team 3);

● Anticipate the possible evolution of the coastal ecosystem in response to global change, by developing scenarios describing the response of organisms and the coastal ecosystem to the interaction of different facets of global change: climate change, change in farming practices evolution (natural or not) of invasive species (Axis 1 of team 3, strong links to develop with the AR5 of team 2), and

● to develop a transdisciplinary approach allowing the co-construction of these scenarios and their analysis with the concerned actors, with a view to decision support in the sustainable management of the coastal socio-ecosystem (links with the “Rade de Brest” axis And with the “unruly” axis, links with the other components of the IUEM).